In the field of secondary batteries comprising an alkaline aqueous electrolyte, as a means for eliminating a memory effect (inactivation) caused by phase transition of nickel hydroxide to be used as a positive electrode active material, a method known is auto-discharging a secondary battery compulsorily after setting the battery in a charger (e.g. Japanese Laid-Open Patent Publication No. Hei 7-14613, Japanese Laid-Open Patent Publication No. Hei 8-33221).
In the field of secondary batteries comprising a non-aqueous electrolyte (non-aqueous electrolyte secondary batteries), on the other hand, such a defect as the memory effect is not recognized in materials used for the electrodes thereof. Therefore, in a typical method for charging a non-aqueous electrolyte secondary battery, after a battery is set in a charger, a voltage of the battery is measured and if the measured voltage is smaller than a charge terminal voltage and larger than a low voltage indicative of a short-circuit, charging is immediately started.
In recent years, application of non-aqueous electrolyte secondary batteries as main power sources chiefly for mobile phones has been expanding. With such expanded application of non-aqueous electrolyte secondary batteries, it has been becoming clearer that, when a non-aqueous electrolyte secondary battery with a large remaining capacity is charged, the battery capacity deteriorates in a large degree. The cause of the capacity deterioration has been understood as follows although the details thereof are unclear.
In a lithium-ion secondary battery, a negative electrode is larger in size than a positive electrode. In the case of a lithium-ion secondary battery being stood still in a charged state, therefore, lithium ion having been absorbed in a negative electrode diffuse in a position of the negative electrode, which is not opposed to a positive electrode. The lithium ion thus having diffused are considered as becoming incapable of contributing to discharging (Japanese Laid-Open Patent Publication No. 2003-36891).
Japanese Laid-Open Patent Publication No. 2003-36891 has proposed a method where, in order to suppress deterioration in capacity of a non-aqueous electrolyte secondary battery, measurements of a battery voltage, constant-current discharge and standing-still of the battery after the discharge are repeated, and once the battery voltage becomes not larger than a prescribed voltage, charging is conducted. This method however has the following problem.
In the case where batteries are auto-discharged compulsorily until becoming not larger than a prescribed value in voltage across the board, there may be no problem when a remaining capacity of the battery is small. When a remaining capacity is large, however, the time required for the auto-discharge becomes longer, thereby raising a problem of a longer period of time to complete charging. It is not rare that a secondary battery with a remaining capacity of 80% or larger is set in a charger. In view of usability of customers, the aforementioned method is not practical.
Moreover, across-the-board compulsory auto-discharging naturally causes an increase in number of repetition of charge/discharge cycles. This may consequently induce capacity deterioration based on another factors, such as deterioration in active material. Hence, it becomes impossible to realize improvement of a cycle life characteristic of a battery.
Furthermore, auto-discharging a battery with a large remaining capacity can be regarded as being equivalent to wasting a large amount of energy. Therefore, such a method cannot be recommendable in the modern society with plenty of environmental and energy problems.